Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
Data is typically written to the disk by modulating a write current in an inductive coil to record magnetic transitions onto the disk surface in a process referred to as saturation recording. During readback, the magnetic transitions are sensed by a read element (e.g., a magnetoresistive element) and the resulting read signal demodulated by a suitable read channel. Conventional disk drives have employed a single magnetic recording layer wherein each symbol cell along the length of a data track is magnetized (e.g., longitudinally or perpendicularly) to record a single binary data bit. Three-dimensional magnetic recording (3DMR) is a recent development wherein multiple magnetic recording layers are fabricated on the disk surface each having a different saturation field level. An example of 3DMR is illustrated in FIG. 1 wherein a first magnetic recording layer 21 (e.g., top layer) exhibits a first magnetization (M) versus magnetic field (H) or M-H hysteretic response 41, and a second magnetic recording layer 22 (e.g., bottom layer) exhibits a second M-H hysteretic response 42. In this example, the saturation field level (HS1) of the first magnetic recording layer 21 is greater than the saturation field level (HS2) of the second magnetic recording layer 22, and therefore the first magnetic recording layer may be considered a “hard” layer and the second magnetic recording layer may be considered a “soft” layer. A non-binary symbol may then modulate the amplitude of the write current in order to write a non-binary symbol to each symbol cell along the length of a data track. During readback, the varying magnetization of each magnetic recording level results in a varying amplitude of the read signal that may be demodulated into the recorded non-binary symbol.